1. Field of the Invention
The technology of cleaning with heat has existed for many years and has been practiced in the recovery of both consumer and industrial products. One successful consumer product is the self-cleaning oven which operates on the simple principle of using heat to clean when the oven interior becomes dirty or contaminated from baked-on food spills or grease splattering. The self-cleaning cycle is initiated by bringing the oven temperature up to about 750.degree. to 950.degree. Fahrenheit, substantially higher than the normal cooking termperatures of 250.degree. to 550.degree. Fahrenheit. The higher temperature level which is normally maintained for a suitable time period removes the baked-on organic residues by the combined action of vaporization, thermal decomposition and oxidation. The gaseous products are generally exhausted through a catalytic oxidizing unit before being discharged to the atmosphere. Any residue remaining on the interior walls of the oven is normally removed as a soft ash. The danger of fire or explosion in normal operation of a self-cleaning oven is negligible primarily due to the fact that the volume of combustible pyrolysis smoke and gases emitted is small and the concentration of combustible material in the oven interior and exhaust stream never attains an explosive or ignitable level.
In contrast, fires and explosions have occurred more frequently in early versons of heat-cleaning equipment used in industrial environments. For industrial cleaning problems, the so-called burn-off or reclamation furnaces were among the first widespread applications of using heat to remove combustible or other organic material from metal parts. Burn-off ovens have been accepted and used by industry because they offer a simple economical way to solve many industrial cleaning problems. However, along with their use, such ovens in some cases developed a pattern of somewhat poor performance and lack of confidence in their use because of occasional fires or explosions associated with their normal usage.
Originally, burn-off or reclamation furnaces or ovens did not include much more than a heated, vented chamber into which contaminated parts were loaded and heated to a processing temperature of about 700.degree. to 800.degree. Fahrenheit. As the temperature of the parts became elevated on heating, the organic contaminants decomposed to combustible smoke and vapors. In cases where the loading of combustible material on the parts was excessive or the heat-up rate employed was too fast and venting of the combustible gases inadequate, frequently the combustible gases could be evolved at such rate to ignite and burn in an uncontrolled manner. Normally fires and explosions have occurred where the emission rate of volatile gases from the parts became greater than the enclosure holding the parts or venting equipment could handle and the enclosure became filled with an explosive mixture of combustibles and air. Fires have resulted in warping or other damage to the parts and sometimes to the structure of the furnace or oven itself.
2. Description of the Prior Art
Over many years various improvements have been made in the design and operation of burn-off or similar reclamation furnaces. U.S. Pat. No. 3,839,086 to Larson discloses the method of injecting water spray or vapor into a rotary dryer or furnace used for removing oil from metal scrap or turnings generated during metal-working operations. As the oil was removed in the form of combustible gases, the vaporized oil would initiate a partial combustion reaction which is highly exothermic. A plurality of water injection nozzles were provided which were activated in response to a temperature sensor inside the dryer and successively to the activation of a cam-type arrangement to supply enough water to control the exothermic burning or oxidation of the oil. U.S. Pat. No. 3,544,367 to Ehrlick and Thomas discloses the use of both oil and water injection to control the combustion or oxidation reaction recurring in a metal scrap dryer. Both of the referenced patents disclose water injection as a means of controlling the highly exothermic reaction of the oil as it is vaporized and removed with heat supplied by fuel-fired burners.
U.S. Pat. No. 4,270,898 to Kelly, assigned to the same assignee as the present application, discloses an automatic control system for a reclamation furnace which comprises an automatic means for sensing the level of combustible pyrolysis smoke and gases emitted from the contaminated parts as they are heated up, coupled with a water injection system which is responsive to the smoke emission sensor to cool the furnace interior and parts and maintain the smoke emission rate at safe levels. In this manner, the emission rate of combustible pyrolysis gases emitting from the furnace interior into an afterburner chamber is maintained below a level which could lead to partial combustion, fires or explosions.
This system has some inherent deficiencies in its on-off control of the heat input burner to maintain a pre-selected furnace temperature. During the off cycle of the burner air can leak through the burner into the furnace causing possibly dangerous variations in oxygen content of the furnace atmosphere. Similar conditions can occur when the burner occasionally fails to restart. Also occasional plugging of the water spray nozzle tips can prevent proper furnace cooling.
In addition to the aforesaid methods using heat as the cleaning agent, additional high-temperature cleaning processes or equipment have been developed which employ other means of preventing fires or explosions in apparatus for practicing cleaning processes which emit combustible gases. U.S. Pat. No. 3,936,659 to Mainord discloses a high-temperature oven which employs inert gases to control the oven atmosphere to eliminate danger of fire or explosion while the organic contaminants on the parts are decomposed by heating. An alternate way to prevent such deleterious effects during high-temperature cleaning is to employ a vacuum within the furnace or oven thereby eliminating or reducing the oxygen level in the heating chamber.
The present invention is directed to overcoming the adherent problems of prior art systems, specifically the methods disclosed in the Kelly patent, for preventing fires or explosions in a burn-off or reclamation furnaces, wherein the heat-input burner is cycled on and off responsive to preset temperature conditions.